Induced pluripotent stem cells hold great promise as a source of patient-specific cells in regenerative medicine, but there are many challenges that must be overcome before this technology can be applied effectively in clinical therapies (Hanna et al., Cell 143:508-525, 2010; Yamanaka, Cell 137:13-17, 2009; Stadtfeld et al., Genes Dev 24:2239-2263, 2010). One critical issue is the use of an oncogene, c-Myc (M), together with other three factors KOS (Klf4, Oct4, Sox2) to generate induced pluripotent stem (iPS) cells. The ectopic overexpression of KOS alone leads to a low efficiency of iPS cell formation (Takahashi and Yamanaka, Cell 126:663-676, 2006). However, the use of oncogenes raises serious concern about tumorigenicity of iPS cells and long-term safety in potential clinical use of iPS cells. Indeed, recent reports show that even after successful reprogramming, iPS cells tend to show low genome stability and premature cellular senescence upon differentiation (Feng et al., Cell Stem Cell 4:301-312, 2009; Hu et al., Proc Natl Acad Sci USA 107:4335-4340, 2010). However, increasing cell proliferation and suppressing genome stability by c-Myc seems to be inseparable from efficient induced pluripotent stem cell (iPSC) formation (Zhao et al., Cell Stem Cell 3:475-479, 2008). Thus, a fundamental challenge that must be addressed is how to increase efficiency of iPS cell generation without sacrificing genome stability. Without resolving this issue, iPS cells may never be usable in clinical practice.
The Zscan4 (zinc finger and scan domain-containing protein 4) gene was identified by expression profiling of all preimplantation stages of mouse embryos using a large-scale cDNA sequencing project (Ko et al., Development 127:1737-1749, 2000; Sharov et al., PLoS Biol 1:E74, 2003) and DNA microarray analysis (Hamatani et al., Dev Cell 6:117-131, 2004). In mice, Zscan4 consists of 6 paralog genes (Zscan4a to Zscan4f) and 3 pseudogenes (Zscan4-ps1 to Zscan4-ps3) clustered on an approximately 850 kb region of chromosome 7. Among the six paralogs, the open reading frames of Zscan4c, Zscan4d, and Zscan4f encode a SCAN domain as well as all four zinc finger domains, suggesting their potential role as transcription factors. A high expression peak of Zscan4 marks the late 2-cell stage of mouse embryos. Zscan4 expression, normally below detection threshold in blastocysts, is reactivated in vitro in a small fraction of embryonic stem (ES) cells in culture. It has previously been demonstrated that Zscan4 acts critically in the formation of proper blastocysts (Falco et al., Dev Biol 307:539-550, 2007; PCT Publication No. WO 2008/118957) and is required for the maintenance of genome stability and normal karyotype in ES cells (Zalzman et al., Nature 464:858-863, 2010; PCT Publication No. WO 2011/028880).